The message means what it says - their server, or the server doing the delivery cannot connect to your server. There could be any number of reasons for that, including things like a routing problem. However as the recipient your hands are tied on troubleshooting - it needs to be troubleshooted by the sender.
If the sender routes their email via an ISP then you will struggle to get assistance - most ISPs attitude is that there is nothing wrong with their network (even when there is and evidence can be produced).

Upgrading from older Exchange server to the latest Exchange server can be tiresome, error-prone and risky, without being a seasoned exchange server administrators. It can become even problematic if you're an organization that runs on tight timeline…

This video shows how to quickly and easily deploy an email signature for all users in Office 365 and prevent it from being added to replies and forwards. (the resulting signature is applied on the server level in Exchange Online)
The email signat…

This video tutorial shows you the steps to go through to set up what I believe to be the best email app on the android platform to read Exchange mail.
Get the app on your phone: The first step is to make sure you have the Samsung Email app on your …